Operation of shaft furnaces



' Patented July 21 1942 I UNITED STATES PATENT OFFICE .strxrza izaztma. I No Drawing. Application January 28, 1941, Se-

rial No. 376,273. 1940 2Claims';

The invention relates to the operation of shaft In Germany January 13,

furnaces such as the blast furnaces commonly I used for the production of pig from more particularlyRwhen using a highly heated blast and/or a blast enriched with oxygen. By using oxygenenriched blast or heated blast, the smelting rate of the furnace is increased to an important degree, and substantial savings in coke are made ction ,of hich can possible together with increased pr pig iron; but the proportion of oxyge be used in the blast is limited factors. 1

When oxygen-enriched blast is used, a very high temperature is attained in the hearth and smelting is rapid, but a relatively small volume of highly heated gas is produced. This small volume of very hotgas does-not always properly prepare the charge descending in the furnace.

reducing gases either to the hearth or to the mm or both, but this entails considerable expense in thatspecial gas heaters are necessary.- Moreover,- there is considerable danger involved in heating such gases to the necessary high temperature.

by a umber of part of the gas issuing from the top of the furnace, burning it to a hot mixture of carbon dioxide and nitrogen, and adjusting the temperature of this mixture to such a degree and introducing the mixture into the blast furnace shaft through suitable inlets at such a level that theintroduced carbon'dioxide cannot be reduced by the coke -of the charge when rising upward in the shaft, the introduced "mixture thus preheating the upper portions of the charge to the desired'degree.

Preferably the temperature of the gas admitted is as high as and most desirably slightly higher.

than the temperature of the gas already rising within the furnace at the level of the gas inlets above mentioned. However the gas admitted should not be at a temperature so 'high that'its carbon dioxide content is reduced to carbon monoxide by the coke in the charge, and correspondingly at the level of the gas inlets the temperatures of'the ascending furnace gas and descending coke are so low, that no reduction of carbon dioxide by the 'coke can occur.

If the gas taken from the furnace top and burned is at such a high temperature that its carbon dioxide content would be reduced to carbon monoxide by the coke in the charge, it may be cooled in a suitable manner 1. e. by mixing Still another proposal'which has been ad-- I v'anced is that air be admitted to the shaft of the furnace to burn-at least a portion of the gas rising in the furnace. This proposal is also not practical because to prevent explosions in the fur- 'nace,- the gas rising in the stack must be at ignitidn temperature at the point where the air is to be .admitted. Also, if the gas is burned-in the furnace, very high local temperatures are attained and more coke is consumed.

It is the chief object of the present invention to make possible the use of higher'concentrations of oxygen in the'blast of a blast furnace than have heretofore been possible without encountering the disadvantages formerly met. An-

with it a sufficient quantity of other gases (nitrogen, or flue gas or unburned furnace gas). By

adding a suitable quantity of hot burned gas to the gas already rising in the furnace, the heat content of the furnace gas may be increased to ciency'of heat in the gas rising from the lower parts of the furnace. j

if the total volume of gas rising in the furnace and o admitted gas is so great that it tends to hinder he downward passage of the charge in the. furnace, a portion of the gas in thefurnace, which gas has been cooled by the charge,

may be removed at a pointbelow that at which the recirculated gas is admitted. A portion of the gas withdrawn may be mixed with the gas which comprises utilizing .an oxygen-enriched blast containing oxygen in such proportions as would normally detrimentally. affect the preheatpurposes.

When recirculated gas is admitted to the furnace in accordance with the invention, the charge is thermically well prepared in that portion of the shaft above the point at which the added gas ing of the charge in the furnace, recirculating a 56' is admitted. The further heating of the charge to the temperature at which it is to enter the hearth, and the reduction of the ore, are accomplished by the gas risingfrom the hearth. This gas contains a large proportion of carbon monoxide and is well adapted for reduction. In view of these good reduction conditions in the parts of the furnace below the point of gas admission it is not necessary that the burned and admitted gas have a high content of reducing components i. e. of carbon monoxide; gases with low content of carbon monoxide or without reducing components may be used.

The gas rising from the hearth must have sufficient heat content to heat the lower portions of the charge. Its heat content may be regulated by regulating the temperature of .the blast, theoxygen concentration in the blast, and the proportion of coke in the charge so that a sufficient volume of gas at a sufficient temperature is furnished to preheat the charge in the furnace between the hearth zone and the point at which the burned furnace gas is admitted according to the invention. For example if it is necessary to increase the volume of gas rising from the hearth, the temperature of the blast is raised while the concentration of oxygen in the blast is decreased.

The process of the invention wherein the shaft is heated with burned furnace gas can be employed whenever the shaft furnace is sufficiently supplied with heat in the smelting zone but the gas volume rising in the shaft is not sufficient for preheating the charge. Such conditions are not only encountered when the blast is greatly enriched with oxygen, but also when very high blast temperatures are utilized or when an exceedingly hot blast enriched with oxygen is used.

Large quantities of contained moisture, water of hydration and carbonic acid in the ore, the latter, for example, bound in FeCOz, cause a large heat consumption in the shaft. Ores of this type can therefore be handled advantageously according to the process of the invention wherein the heat content of the gas rising in the furnace is increased.

The new process possesses all the advantages which accompany an abundant oxygen enrichmerit of the blast or high heatingof the blast such as, for example, a great saving in coke, increased smelting velocity, increased production, insensitivity against high free-running temperatures of the resulting slags, high manganese yield, etc. and all these to a greater extent than has heretofore been possible. The process'of the invention makes the use of oxygen-enriched blast or highly heated blast possible to a far greater degree, the disadvantages of these methods of operation, such as possible deficient preparation of the charge prior to its entrance into the smelting zone, being eliminated.

In addition the new process permits all the produced furnace gas to be consumed within the scope of the blast furnace operation itself, that is, all of the fuel used in the blast furnace is biu'ned completely to carbon dioxide. This is of particular importance when only remelt pig iron is to be produced using, on an ore'basis, expensive coke, and when there is not connected with the blast furnace works the steel mill and rolling mill operations which usually consume the excess furnace gas. Heretofore, where there was no use for excess furnace gas, the problem of attaining an equalized furnace gas economy presented the greatest difficulties, and it was nesessary to provide expensive auxiliary plants which consumed power and in addition were foreign to blast furnace operations.

In the process of the present invention, however, there is produced, first of all, less carbon monoxide because as a result of the oxygen enrichment or the increased blast temperature less coke is consumed; and secondly a large part of the small volume of furnace gas produced is burned before being blown into the blast furnace shaft, while a further portion is consumed for the oxygen production or the increased blast heating. Despite reduced blast volumes in the process of the invention, the quantity of furnace gas consumed for the blast heating is not less than with air operation when the highest blast temperatures are utilized in conjunction with the oxygen-enrichment. It is also worthy of mention that the furnace gas need not be purified before it is recirculated, nor are special gas heaters for the recirculated gas necessary.

One fundamental advantage of the new process is that the heat supply to the preparatory zone in the shaft and to the smelting zone in the hearth of the furnace can be controlled independently of one another through the quantity and temperature of the inlet gas, oxygen concentration and temperature of the blast. In customary blast furnace operations the only control is through blast temperature and coke charge and is very sluggish.

The reduction processes in the hearth can be influenced through variation in the available heat in the shaft.- For example, with ample preheating of the charge and with high gas temperatures, high manganese or high silicon pig iron would be obtained, the latter particularly easily when running an acid slag. High silicon pig iron is particularly suitable for, among other things, use a remelt pig iron because during the remelting in the blast furnace the re-oxidation of the silicon replaces a portion of the necessary smelting coke.

However, the present process can also be utilized for producing a low silicon pig iron, for example, Thomas pig iron, in that the charge in the shaft is heated to a lesser degree, so that the heat supply in the furnace hearth is opposed by a greater heat requirement for the direct reduction as well as for the warming and smelting, so that less heat remains for the silicon reduction. According to the above, the process is well suited for the smelting of acid ores, poor in iron which, without previous preparation, can be processed to remelt pig iron or even to Thomas pig iron with the lowest coke consumption and with complete utilization of the furnace gases for blast furnace operation purposes.

The construction of such a smelting works,

operating with a balanced furnace gas economy, becomes extremely simple because it requires no preparatory plants nor roasting plants and no auxiliary plants for utilizing the excess furnace gas. The blast furnace is only equipped in the upper part of the shaft with a series of furnace gas burners with gas mixing chambers.

The new process is not limited to the smelting of iron ores in the blast furnace. This method of operation can also be employedwith the described advantages, for smelting other metals which are produced in a shaft furnace with coke or other solid fuels.

If a new blast furnace is to be constructed, the advantages of the invention may be attained by increasing the diameter of the shaftabove the point at which the recirculated gas is admitted.

enriched blast, recirculating at least a portion 10 of the gas produced in the furnace and readmitting such gas to the furnace at a point above the level of the blast but well below the top of the charge in the furnace, the improvement which comprises burning the said portion of furnace 15 gas before it is re-admitted to the furnace.

2. In a method of smelting a charge comprising oxydic ore and carbonaceous reducing agent in a blast furnace, which method comprises subiecting the charge to the action of an oxygen- 5 enriched blast, recirculating at least a portion of the gas produced in the, furnacefand readmitting such gas to the furnace at a point above the level of the blast but well below the top of the charge in the furnace, the improvement which comprises burning the said portion of furnace gas before it is re-admitted to the furnace and adjusting the temperature of such burned gas I so that carbon dioxide contained in thevgas is not reduced by the furnace charge. 4

ERNST KARWAT. 

